different data for the same station on wunderground.com and station

I found difference in temperatures when compare data for many stations on the http://www.wunderground.com site and data from the same stations on Russian weather sites.
I believe data on Russian sites are correct because they are coinciding for the same stations.

Why is there difference in data if wunderground.com clame:” International current conditions are collected directly from more than 29,000 weather stations located in countries around the globe including:”

wunderground.com rounded temperatures: for example station report temperature in Moscow -3,4C at 9p.m. wunderground.com show -4.0C for the same station at 10p.m.